As mentioned in the comments here is a quick hint to placing a head in a jar...

1. you will need the following pieces - a glass jar, a head piece, a plate 1x1 round (optional), plate 2x2 round.

2. place the 1x1 round in the very centre of the 2x2 round

3. place the head on the 1x1 round

4. place the glass jar over the 2x2 round.

A plate 2x2 square, brick 016 w cross, brick 2x2 or any combination of four studs can replace the 2x2 plate as the base. The 1x1 plate round is optional, so long as the head is dead centre the jar will fit over the top.

Quoting Farhan Salman
Awesome. Hey, how come you can put the head in the jar? I've tried but it cannot fit in LDD. Anyway, I like it.

There is a knack to it which I just realised.
First you need to place the head between four studs, a 2x2 plate or 2x2 brick will work, the head needs to be dead centre between all 4 studs.
then place the glass case over the four studs.
I'll add some pics to help explain a little better.